Best use of cocoa - Bronze award

Daniel Vik
daniel@vik.cc
http://danielvik.com/

Judges' comments:

To build:

make vik

This entry requires Zlib.

To run:

Embed text (from file or command line):

./vik e base.png filename > encodedimg.png
# or
./vik e base.png SomeRandomText > encodedimg.png

Extract the embedded text:

./vik d encodedimg.png

Embed a PNG into another

./vik e base.png source.png > encodedimg.png

Extract the embedded PNG:

./vik d encodedimg.png > restored.png

Try:

./vik e ioccc.png chocolate.png > chocolate-in-ioccc.png
./vik e chocolate.png ioccc.png > ioccc-in-chocolate.png
./vik d chocolate-in-ioccc.png > restored-chocolate.png
./vik d ioccc-in-chocolate.png > restored-ioccc.png

Selected Judges Remarks:

References to chocolate had no effect on judging this entry. We guarantee it.

Can you discern hidden pieces of chocolate in chocolate-in-ioccc.png ?

Author’s comments:

Introduction

This program is a steganography application for embedding an image or text into another image as well as extracting the embedded image or text back. The program stores the embedded image or text in the low bits of the RGB values.

The program supports any 8 bit true color PNG images (RGB, RGBA, grayscale, and grayscale+alpha). The output image is always 8 bit RGB. When embedding one image into another, the width and height needs to be the same.

The program only updates the IDAT and IHDR chunks of the source image. Any additional chunks are copied into the resulting image.

Bonus Extractor:

For some reason the chocolate image seems to have some special properties. Apart from being quite big and a little bit noisy, it appears that when embedded into another image and extracted, the bitmap data is also a valid brain$#@$ program. It is of course possible to get the bitmap data from the extracted image, and run it through any of the previous winning brain$#@$ interpreters, but I thought it would be easier to include an interpreter in the program to avoid the hassle:

./vik e ioccc.png chocolate.png > encodedimg.png
./vik b encodedimg.png

It is of course also possible to embed a brain$#@$ program as a text file (as explained above) and decode it, e.g.:

./vik e ioccc.png .+[.+] > encodedimg.png
./vik b encodedimg.png

Obfuscation

I really wanted to keep the program simple, so instead of adding multiple macros or helper methods that can be confusing, I placed all functionality in the main function. Main is called recursively and quite extensively, but it gives the benefit that all invocations of the function has the same parameters, argv and argc. This really helps readability, as a programmer doesn’t need to remember several variable or function names. I also tried to reduce the number of keywords, and the program only has four for-loops followed by a single return statement. There is a little bit of use of the question mark operator, but this is really there to keep the program as simple as possible.

Portability

The program is portable to most platforms that have zlib available. The only system dependency is that the program relies on writing binary data to stdout.

By default, Microsoft compilers adds carriage returns to new lines, and to compile the program on this platform, the following line can be added after the variable declarations in the main declaration in order to run correctly:

_setmode(_fileno(stdout), 0x8000);

Limitations

The program doesn’t have many error checks so passing in invalid arguments or images of mismatching sizes, or unsupported pixel formats will cause the program to crash.

Extendability

Since a lot of care was taken to keep the code simple, it turned out to be quite easy to extend functionality. I did include a small brain$#@$ after I realized that the chocolate image had some interesting properties.

But there is more functionality I added that didn’t fit within the size limits and could not remain included. For example, I added a method to format source code based on a PNG image. So the format of the program is actually done by the program itself.

And to be honest, the chocolate image did not have a brain$#@$ program embedded to begin with. I added functionality to the program to embed a brain$#@$ into a PNG image and used it to create the image provided with the entry.

I also added a method to analyze PNG images, to print the size and format, as well as the scan line filters.

All these features were quite easy to add, much thanks to the (actually pretty good) design of the code.